Risers are tubular structures which extend from a subsea wellhead to a floating structure and are used both for drilling through and producing from subsea wellheads. Such risers are subjected to deflection because of the movement of the floating structure responsive to wind, waves, tide and currents at the surface and to movement of the riser below the surface responsive to currents. The risers also are subjected to external and internal hydrostatic pressures and axial tension applied from the floating structure to prevent buckling of the riser, to reduce the stresses in the riser and to reduce the bending moment on the wellhead.
Various approaches have been suggested to provide risers with improved performance. Some have had articulated or resilient joints. (U.S. Pat. Nos. 3,168,334 and 3,189,372 to G. D. Johnson). Some have suggested a flexible riser (U.S. Pat. No. 3,424,253 to W. C. Triplett and U.S. Pat. No. 3,538,238 to J. Delacour et al) others have suggested caissions which extend from the bottom to above the surface and taper upwardly (U.S. Pat. No. 3,426,859 to W. F. Manning).
The G. W. Morgan U.S. Pat. No. 3,605,413 issued Sept. 20, 1971 discloses a riser having a varying rigidity in its lower section so that it is the stiffest at the subsea wellhead and increases in flexibility in the upward direction. The lower section of this riser includes a plurality of sections of gradually reduced thickness. This lower section is cantilevered to the wellhead.
All of these prior structures have disadvantages of being excessively expensive, create excessive flexural loads at the wellhead or require excessively close control of the tension on the riser.